“They’re great when you don’t have time to type out entire messages but don’t want to be rude, either,” said Motoko Kondo, 34, who works at a Tokyo design agency. “I sometimes carry out entire conversations with only stickers.”

Line is among a growing number of apps jostling for control of the rapidly growing mobile messaging business. Such startups are no longer just competing for users and their time. They are racing to define how a younger generation — one that has increasingly moved away from traditional text messaging, Facebook, and Twitter — will message on the go.

“There’s a frenzy of experimentation going on with the future of messaging,” said Benedict Evans, a partner at the venture capital firm Andreessen Horowitz, which has invested in social networks and messaging services.

Apps like Snapchat and Blink have increased their user bases through the appeal of spontaneity, transience, and privacy, with messages that self-destruct soon after they are opened. Those services have eaten into the early lead of WhatsApp, a no-frills service that focuses on letting users send and receive text, images, audio, and video via the Web. Viber has focused on messaging plus free Internet phone calls.

The Tokyo startup Line, along with WeChat from China, has sought to pepper the experience with stickers, social games, and even weather forecasts. Their sprawling online offerings make them huge distribution platforms for content, not just messaging services.

Line has signed up 430 million users, almost 90 percent of them outside Japan. (It does not break out the number of active users.) WhatsApp said in February that it had 465 million monthly active users. WeChat’s parent company, Tencent, said its number of monthly active users reached 355 million at the end 2013.

Such apps have been the target of frenzied dealmaking. Snapchat spurned a multibillion-dollar offer from Facebook last year; Facebook bought WhatsApp in February for $19 billion. The same month, Rakuten, the Japanese e-commerce giant, acquired Viber for $900 million. Yahoo snapped up Blink this month for an undisclosed sum.

Analysts speculate that Line could go public or be an acquisition target. Justin Lee, a technology analyst at BNP Paribas, values the service at as much as $15 billion.

The question is whether newcomers like Line will translate to a US audience.

“A lot of the Asian platforms have developed much deeper, richer experiences, and I personally think these broader platform approaches have a bigger potential over time,” said said Boris Wertz, founder of Version One Ventures, a venture capital firm. “But that richer experience makes it harder to translate across cultures. Line’s heavy use of stickers and emoticons — you couldn’t transport all of that to North American culture.”

Line is finding it difficult to gain traction in the United States. After an initial marketing push last year, it has eased off promotional activities and says it is focusing on other markets that need attention more urgently.

Line has struck a chord in Japan partly because of the country’s affection for all things cute, experts say. But there are more factors in play. The stiff, written form of the Japanese language can be cumbersome to type on smartphones, and stickers make communication easier and faster, said Ryoko Morishima, who has written several guides on social media.

Stickers also let Japanese users express direct feelings that can be awkward to convey in the formal language, Morishima said. Refusing an invitation from a colleague for after-work drinks, for example, could cause discomfort in a culture where an outright no is avoided at all costs. But enlist a bear as the messenger and there is less potential for embarrassment on either side, Morishima said.

“It’s just easier when a bear says it,” she said.

Line’s characters are not just about better messaging, either. Although Line is free to download, it sells premium-edition stickers, and the characters populate its universe of online games — both big moneymakers for the company. There is the merchandising, as well as a line of peripheral apps, including a weather service, a fledgling e-commerce site, and a dedicated camera app that allows users to add characters to their photos.

Companies can pay to have Line offer their characters, which users download at no cost in exchange for “friending” the corporation in the app. One of the most popular stickers in Japan this year is Rakuten Cardman, a character introduced by the eponymous e-commerce giant. The character recently threw the first pitch at a baseball game hosted by the Tohoku Rakuten Golden Eagles, the former team of New York Yankees pitcher Masahiro Tanaka.

All that has allowed Line to build a solid business model. In the 12 months through March, Line earned almost $590 million from its stickers, in-game sales, and ads, just under the $665 million that Twitter earned last year.

“Line has become part of the communications infrastructure,” said Kenichi Sugai, a technology analyst at Speeda, a financial information platform in Tokyo. “Now it’s in a global race to win over users and to convince them this is what messaging should be.”